A1 adenosine receptor antagonists

ABSTRACT

A compound useful as an A 1  adenosine receptor antagonist has formula (I), wherein R 1  is selected from the group consisting of C 1 -C 8 alkyl; R 2  is of formula (II), wherein n is an integer ranging from 1 to 8; R 5  is H or CH 3 (CH 2 ) p , wherein p is an integer ranging from 1 to 7; and R 6  is H; (CH 2 ) m H; or (CH 2 ) m OH, wherein m is an integer ranging from 1 to 8; R 3  is selected from the group consisting of: (a), (b), (c) and (d), wherein q is an integer ranging from 1 to 8; D is selected from the group consisting of NH, S, and O; wherein R 7  is selected from the group consisting of H, OH, NH 2 , R 9 COOH, wherein R 9  is an alkylene or alkenylene group having 1 to 8 carbon atoms, and (CH 2 ) t OH, wherein t is an integer ranging from 1 to 8; R 11  is selected from the group consisting of —CH 2 COOH and —CH 2 —CONH(CH 2 ) w NHZ, wherein w is an integer ranging from 1 to 2 and Z is selected from the group consisting of hydrogen and acetate; and R 4  is of formula (III), wherein R 8  is selected from the group consisting of H; OH; (CH 2 ) f NH 2  wherein f is selected from the group consisting of 0 and an integer ranging from 1 to 8; (CH 2 ) 5 OH, wherein s is an integer ranging from 1 to 8; and R 10 COOH, wherein R 10  is an alkylene or alkenylene group having 1 to 8 carbon atoms; and r is an integer ranging from 1 to 8.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The instant application is a continuation-in-part application of U.S. patent application Ser. No. 08/753,048 filed Nov. 19, 1996.

FIELD AND BACKGROUND OF THE INVENTION

[0002] The present invention relates to novel compounds useful as A₁ adenosine receptor antagonists.

[0003] Adenosine receptors are involved in a vast number of peripheral and central regulatory mechanisms such as, for example, vasodilation, cardiac depression, inhibition of lipolysis, inhibition of insulin release and potentiation glucagon release in the pancreas, and inhibition of neurotransmitter release from nerve endings.

[0004] In general, adenosine: receptors can be divided into two main classes, A₁ receptors which (,an inhibit, and A₂ receptors which can stimulate adenylate cyclase activity. One of the best known classes of adenosine receptor antagonists are the xanthines which include caffeine and theophylline. See e.g., Müller et al., J. Med. Chem. 33: 2822-2828 (1990). In general, many of these antagonists often suffer from poor water solubility, and low potency or lack of selectivity for adenosine receptors. Additionally, selective analogues of adenosine receptor antagonists have been developed through the “functionalized congener” approach. Analogues of adenosine receptor ligands bearing functionalized chains have been synthesized and attached covalently to various organic moieties such as amines and peptides. Attachment of the polar groups to xanthine congeners has been found to increase water solubility. Nonetheless, such developments have yet to fully address problems associated with potency and selectivity. More recently Jacobson et al. J. Med. Chem. 35: 408-422 (1992) has proposed various derivatives of adenosine and theophylline for use as receptor antagonists. The article discloses that hydrophobic substituents are able to potentially enhance affinity. However, it is also acknowledged that such substituents may result in a decrease in solubility thus rendering the antagonists less soluble in vivo. In confronting these problems, Jacobson et al. indicates that a dipropyl substitution at the 1 and 3 positions of theophylline allows desirable affinity at A₁ receptors. It is also stated that substitutions at the 7-position are typically not favorable.

[0005] It is an object of the present invention to therefore provide compounds useful as Al adenosine receptor antagonists which display high potency and affinity levels, along with water solubility.

SUMMARY OF THE INVENTION

[0006] In one aspect, the present invention provides a compound of the general formula:

[0007] wherein

[0008] R₁ is selected from the group consisting of C₁-C₈ alkyl;

[0009] R₂ is of the formula:

[0010] wherein n is an integer ranging from 1 to 8; R₅ is H or CH₃(CH₂)_(p), wherein p is an integer ranging from 1 to 7; and R₆ is H, (CH₂)_(m)H, or (CH₂)_(m)OH, wherein m is an integer ranging from 1 to 8;

[0011] R₃ is selected from the group consisting of:

—(CH₂)_(q)C₆H₄—R₇

[0012] wherein q is an integer ranging from 1 to 8; D is selected from the group consisting of S, NH, and O; and wherein R₇ is selected from the group consisting of H, OH, NH₂, R₉COOH, wherein R₉ is an alkylene or alkenylene group having 1 to 8 carbon atoms, and (CH₂)_(t)OH, wherein t is an integer ranging from 1 to 8; wherein R₁₁ is selected from the group consisting of —CH₂COOH and —CH₂—CONH(CH₂)_(w)NHZ, wherein w is an integer ranging from 1 to 2 and Z is selected from the group consisting of hydrogen and acetate; and

[0013] R₄ is of the formula:

[0014] wherein R₈ is selected from the group consisting of H; OH; (CH₂)_(f)NH₂ wherein f is selected from the group consisting of 0 and an integer ranging from 1 to 8; (CH₂)_(s)OH, wherein s is an integer ranging from 1 to 8; and R₁₀COOH, wherein R₁₀ is an alkylene or alkenylene group having 1 to 8 carbon atoms; and r is an integer ranging from 1 to 8.

[0015] In a second aspect, the invention provides for assay-type probes of the above compound, wherein the probes are marked or conjugated with radioactive or non-radioactive material.

[0016] In a third aspect, the invention provides a pharmaceutically acceptable salt of the above compound.

[0017] In a fourth aspect, the invention provides a pharmaceutical composition which comprises the above compound and a pharmaceutically acceptable carrier.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention will now be described more fully hereinafter, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0019] The present invention is directed to a compound of the formula (I):

[0020] R₁ is selected from the group consisting of C₁-C₈ alkyl, preferably C₁ to C₄ alkyl. For the purposes of the invention, R₁ is more preferably C₁ or C₃ alkyl, and is most preferably C₃ alkyl.

[0021] R₂ is of the formula:

[0022] wherein n is an integer ranging from 1 to 8, more preferably 1 to 4; R₅ is H or CH₃(CH₂)_(p), wherein p is an integer ranging from 1 to 7, more preferably 1 to 4; and R₆ is H, (CH₂)_(m)H, or (CH₂)_(m)OH, wherein m is an integer ranging from 1 to 8, more preferably 1 to 4.

[0023] R₃ is selected from the group consisting of:

—(CH₂)_(q)C₆H₄—R₇

[0024] wherein q is an integer ranging from 1 to 8; D is selected from the group consisting of S, O, and NH; and wherein R₇ is selected from the group consisting of H, OH, NH₂, R₉COOH, wherein R₉ is an alkylene or alkenylene group having 1 to 8 carbon atoms, and (CH₂)_(t)OH, wherein t is an integer ranging from 1 to 8. The alkylene or alkenylene groups may be substituted or unsubstituted. R₉ is preferably CH═CH. R₁₁ is selected from the group consisting of —CH₂COOH and —CH₂—CONH(CH₂)_(w)NHZ, wherein w is an integer ranging from 1 to 2 and Z is selected from the group consisting of hydrogen and acetate.

[0025] R₄ is of the formula:

[0026] wherein R₈ is selected from the group consisting of H; OH; (CH₂)_(f)NH₂, wherein f is selected from the group consisting of 0 and an integer ranging from 1 to 8; (CH₂)_(s)OH, wherein s is an integer ranging from 1 to 8, more preferably 1 to 4; and R₁₀COOH, wherein R₁₀ is an alkylene or alkenylene group having 1 to 8 carbon atoms; and r is an integer ranging from 1 to 8, more preferably 1 to 4. In the above, R₉ and R₁₀ are preferably CH═CH.

[0027] The invention may be illustrated below with respect to preferred embodiments. In these embodiments, R₃ is of the formula:

—(CH₂)_(q)C₆H₄—R₇

[0028] In one preferred embodiment, R₁ is C₃ alkyl; R₅ is CH₃ (CH₂)_(p) wherein p is 1; R₆ is (CH₂)_(m)OH wherein m is 2; R₇ is H; R₈ is NH₂; f is 0; n is 2; m is 2; q is 1; and r is 2.

[0029] In another preferred embodiment, R₁ is C₃ alkyl; R₅ is CH₃ (CH₂)_(p). wherein p is 1; R₆ is H; R₇ is NH₂; R₈ is NH₂; f is 0; n is 2; q is 1; and r is 2.

[0030] In another preferred embodiment, R₁ is C₃ alkyl; R₅ is CH₃ (CH₂)_(p) wherein p is 1; R₆ is H; R₇ is H; R₈ is NH₂; f is 0; n is 2; q is 1; and r is 2.

[0031] In another preferred embodiment, R₁ is C₃ alkyl; R₅ is CH₃ (CH₂)_(p) wherein p is 1; R₆ is H; R₇ is H; R₈ is selected from the group consisting of (CH₂)_(s)OH, wherein s is 2 and R₁₀COOH, wherein R₁₀ is CH═CH; n is 2; q is 1; and r is 2.

[0032] In another preferred embodiment, R₁ is C₃ alkyl; R₅ is CH₃ (CH₂)_(p) wherein p is 1; R₆ is H; R₇ is selected from the group consisting of R₉COOH, wherein R₉ is CH═CH and (CH₂)_(t)OH, wherein t is 2; R₈ is NH₂; f is 0;n is 2; q is 1; and r is 2.

[0033] The compound of the present invention may form pharmaceutically acceptable salts with both organic and inorganic acid and bases. Examples of suitable acids for salt formation are hydrochloric, sulfuric, phosphoric, acetic, citric, oxalic, malonic, salicylic, ascorbic, maleic, methanesulfonic, and the like. Any of the amine acid addition salts may also be used. The salts are prepared by contacting the free base form of the compound with an appropriate amount of the desired acid in a manner known to one skilled in the art. Examples of suitable bases for salt formation are sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, calcium hydroxide, ammonia, organic amines, and the like. The salts may be prepared by contacting the free acid form of the compound with an appropriate amount of the desired base in a manner known to one skilled in the art.

[0034] The invention also provides A₁ adenosine receptor antagonist compounds with radioactive or non-radioactive labels. Such labelled compounds are useful as assay-type probes or conjugates, and may be used to obtain quantitative binding measurements of the A₁ adenosine receptor antagonist compounds. For the purposes of the invention, “assay-type probes” refers to those materials which are useful for enhancing the selectivity of the quantitative analysis of the A₁ adenosine receptor compounds of the invention. Examples of such assay-type probes are described in U.S. Pat. No. 5,248,770 to Jacobson et al., the disclosure of which is incorporated herein by reference in its entirety. The probes are highly useful in that they have little adverse effect on the affinity of the compounds of the present invention. Radioactive markers include, but are not limited to, an electric spin marker, a ¹⁹F NMR probe, a radioactive ¹⁸F isotope marker, a radioactive iodine marker (e.g., ¹²⁵¹I), a radioactive ³H marker, tritium, and a complex of a metal atom or a metal ion and a chelating agent. An exemplary metal ion is a radioactive isotope of technetium or indium. An exemplary chelating agent is diethylene pentacetic anhydride.

[0035] Various non-radioactive materials may be used in labelling the present A₁ adenosine receptor compounds. Numerous examples are presented in U.S. Pat. No. 5,248,770 to Jacobson et al. Biotin is used-as a common non-radioactive label for such probes, as described in R. W. Old et al. Principals of Gene Manipulation, 4th ed: 328-331 (1989). To facilitate labelling the compounds with biotin or any other appropriate material, a spacer component may be added to the compound according to an accepted method. Such a method is described in the Jacobson et al. '770 patent. Exemplary spacer components include, but are not limited to, an oligopeptide, triglycidyl, and N-hydroxysuccinimide ester.

[0036] Biotin may be bonded to any suitable linkage provided by substituents on the compound structure in accordance with any accepted and suitable technique. For example, referring to compound (I) as defined herein, biotin may be bonded to the hydroxy group on R₆ when the compound contains (CH₂)_(m)OH at R₆ with m defined herein; to the amino group present on either of R₇ or R₈ when (CH₂)_(f)NH₂ is contained at the R₈ position, wherein f is defined herein; to the hydroxyl group present as R₇ or R₈; or to the carboxyl group present when R₇ and R₈ are R₉COOH or R₁₀COOH respectively, with R₉ and R₁₀ defined herein. Additionally, the biotin may be bonded to a hydroxyl group present on R₈, when R₈ is (CH₂)_(s)OH with s being defined herein. Biotin may also be bonded to R₇, when R₇ is (CH₂)_(t)OH with t being defined herein. The biotin-labeled probes may be detected through appropriate and known analytical techniques.

[0037] Fluorescent dyes may also be employed as a non-radioactive labels and are applied to appropriate locations on the compounds of the invention. Such dyes include, but are not limited to, tetramethylrhodamine, fluorescein isothiocyanate, and mixtures thereof. Other non-radioactive materials include for example, nitrobenzoxadiazole; 2,2,6,6-tetramethyl-piperindinyloxy-4-isothiocyanate; and mixtures thereof.

[0038] The invention is also directed to a pharmaceutical composition which includes the compound of the present invention and a pharmaceutically acceptable carrier. The pharmaceutical composition is particularly useful in applications relating to organ preservation in vivo or in situ, perfusion of an isolated organ either removed or contained within the body (e.g., when an organ is transported for transplantation), cardiopulmonary bypass, perfusion of an extremity or limb, and the like. The compounds may be used in intra-articular, intra-thecal, gastrointestinal, and genital urinary applications, as well as in any cavity or lumen such as, for example, the thoracic cavity or ear canal.

[0039] The pharmaceutical composition may be employed, as an example, in oral dosage form as a liquid composition. Such liquid compositions can include suspension compositions or syrup compositions and can be prepared with such carriers as water; a saccharide such as sucrose, sorbitol, fructose, and the like; a glycol such as polyethyleneglycol, polypropyleneglycol, and the like; an oil such as sesame oil, olive oil, soybean oil, and the like; an antiseptic such as p-hydroxy-benzoic acid esters and the like; and a flavor component such as a fruit flavor or a mint flavor. The pharmaceutical composition may also be in the form of powder, pills, capsules, and tablets and can be prepared with various carriers. Suitable carriers include, but are not limited to, lactose, glucose, sucrose, mannitol, and the like; disintegrators such as starch, sodium alginate, and the like; binders such as polyvinyl alcohol, hydroxypropyl cellulose, gelatin, and the like; surfactants such as, for example, fatty acid esters; and plasticizers such as, for example, glycerins. The composition of the present invention is especially useful when applied sublingually. It should be noted that in the preparation of the tablets and capsules, a solid pharmaceutical carrier is used. Advantageously, the pharmaceutical composition may be used in the form of, for example, eye drops or an aerosol.

[0040] Other types of pharmaceutical compositions may be employed in the form of a suppository, a nasel spray, and an injectable solution. These compositions are prepared using appropriate aqueous solutions which may include, but are not limited to, distilled water, and saline and buffer additives. Other components may be employed such as organic materials including neutral fatty bases. Additionally, the pharmaceutical composition may be utilized in a transdermal application.

[0041] Biopolymers may be used as carriers in the above pharmaceutical compositions. Exemplary biopolymers may include, for example, proteins, sugars, or lipids.

[0042] The A₁ receptor antagonists of the present invention are particularly useful as, for example, anti-allergenics, CNS stimulants, diuretics, anti-asthmatics, and cardiotonics.

[0043] Selective analogues of adenosine receptor antagonists have been developed through the “functionalized congener” approach. See e.g., U.S. Pat. No. 4,968,672 to Jacobson et al.; and Jacobson et al., Mol. Pharmacol. 29: 126-133 (1985). In terms of pharmacology, the compounds advantageously display increased affinity at A₁ receptor sites relative to former A₁ receptor antagonists while simultaneously exhibiting good water solubility.

[0044] The foregoing example is illustrative of the present invention, and is not to be construed as limiting thereof.

EXAMPLE Synthesis of A₁ Adenosine Receptor Antagonists

[0045] A₁ adenosine receptor antagonists of the present invention may be synthesized according to the process illustrated below:

[0046] In the above reaction pathway, R′ may be C₁-C₈ alkyl; R″ may be selected from the group consisting of H, OH, (CH₃)_(e)NO₂ wherein e is selected from the group consisting of 0 and an integer ranging from 1 to 8; (CH₂)_(s)OH, wherein s is an integer ranging from 1 to 8; and R₁₀COOH, wherein R₁₀ is an alkylene or alkenylene group having 1 to 8 carbon atom; R^(x) may be selected from the group consisting of:

(CH₂)_(q)C₆H₄R′″

[0047] wherein R′″ may be selected from the group consisting of H, OH, NO₂, R₉COOH, wherein R₉ is an alkylene or alkenylene group having 1 to 8 carbon atoms, and (CH₂)_(t)OH, wherein t is an integer ranging from 1 to 8; D may be selected from the group consisting of O, S, and NH; q is an integer ranging from 1 to 8; R₁₁ is selected from the group consisting of —CH₂COOH and —CH₂—CONH(CH₂)_(w)NHZ, wherein w is an integer ranging from 1 to 2 and Z is selected from the group consisting of hydrogen and acetate; and R^(iv) may be selected from the group consisting of H, (CH₂)_(m)H, and (CH₂)_(m)OH, wherein m is an integer ranging from 1 to 8. As identified in formula (VII), R^(v) may be selected from the group consisting of H; OH; (CH₂)_(f)NH₂ wherein f is selected from the group consisting of 0 and an integer ranging from 1 to 8; (CH₂)_(s)OH, wherein s is an integer ranging from 1 to 8; and R₁₀COOH, wherein R₁₀ is an alkylene or alkenylene group having 1 to 8 carbon atoms. R^(xi) may be selected from the group consisting of:

(CH²)_(q)C₆H₄R^(vi)

[0048]

[0049] wherein D may be selected from the group consisting of O, S, and NH; q is an integer ranging from 1 to 8; R^(vi) may be selected from the group consisting of H, OH, NH_(2,) R₉COOH, wherein R₉ is an alkylene or alkenylene group having 1 to 8 carbon atoms, and (CH₂)_(t)OH, wherein t is an integer ranging from 1 to 8. R₁₁ is selected from the group consisting of —CH₂COOH and —CH₂—CONH(CH₂)_(w)NHZ, wherein w is an integer ranging from 1 to 2 and Z is selected from the group consisting of hydrogen and acetate. In general, the above synthesis steps may be carried out at standard temperature and pressure conditions, optionally under reflux. An exception to this pertains to the reaction involving intermediates (VI) and (VII) which is preferably performed at a temperature ranging from about 25° C. to about 50° C and at atmospheric pressure. In the reaction step involving intermediate (V) becoming intermediate (VI), it is desired to employ an oxidation step with FeCl₃ or NaIO₄ subsequent to the nitrobenzene reflux. Moreover, it should be noted that when R″ and/or R′″ contain nitro groups, a reaction step which involves applying H₂ over a Pd/C catalyst is employed prior to the reaction with CH₃OH/HCl. The resulting product (VII) may be further processed and purified according to accepted procedures.

[0050] In the specification and example, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation of the scope of the invention being set forth in the following claims. 

That which is claimed is:
 1. A compound of the formula:

wherein R₁ is selected from the group consisting of C₁-C₈ alkyl; R₂ is of the formula:

wherein n is an integer ranging from 1 to 8; R₅ is H or CH₃ (CH₂)_(p), wherein p is an integer ranging from 1 to 7; and R₆ is H; (CH₂)_(m)H; or (CH₂)_(m)OH, wherein m is an integer ranging from 1 to 8; R₃ is selected from the group consisting of: —(CH₂)_(q)C₆H₄—R₇

wherein q is an integer ranging from 1 to 8; D is selected from the group consisting of NH, S, and O; wherein R₇ is selected from the group consisting of H, OH, NH₂, R₉COOH, wherein R₉ is an alkylene or alklenylene group having ₁ to 8 carbon atoms, and (CH₂)_(t)OH, wherein t is an integer ranging from 1 to 8; R₁₁ is selected from the group consisting of, —CH₂COOH and —CH₂—CONH(CH₂)_(w)NHZ, wherein w is an integer ranging from 1 to 2 and Z is selected from the group consisting of hydrogen and acetate; and R₄ is of the formula:

wherein R₈ is selected from the group consisting of H; OH; (CH₂)_(f)NH₂ wherein f is selected from the group consisting of 0 and an integer ranging from 1 to 8; (CH₂)_(s)OH, wherein s is an integer ranging from 1 to 8; and R₁₀COOH, wherein R₁₀ is an alkylene or alkenylene group having 1 to 8 carbon atoms; and r is an integer ranging from 1 to
 8. 2. The compound according to claim 1, wherein R₁ is C₃ alkyl; R₃ is: —(CH₂)_(q)C₆H₄—R₇ R₅ is CH₃ (CH₂)_(p) wherein p is 1; R₆ is (CH₂)_(m)OH wherein m is 2; R₇ is H; R₈ is NH₂; f is 0; n is 2; m is 2; q is 1; and r is
 2. 3. The compound according to claim 1, wherein R₁ is C₃ alkyl; R₃ is: —(CH₂)_(q)C₆H₄—R₇ R₅ is CH₃ (CH₂)_(p) wherein p is 1; R₆ is H; R₇ is NH₂; R₈ is NH₂; f is 0; n is 2; q is 1; and r is
 2. 4. The compound according to claim 1, wherein R₁ is C₃ alkyl; R₃ is: —(CH₂)_(q)C₆H₄—R₇ R₅ is CH₃ (CH₂)_(p) wherein p is 1; R₆ is H; R₇ is H; R₈ is NH₂; f is 0; n is 2; q is 1; and r is
 2. 5. The compound according to claim 1, wherein R₁ is C₃ alkyl; R₃ is: —(CH₂)_(q)C₆H₄—R₇ R₅ is CH₃ (CH₂)_(p) wherein p is 1; R₆ is H; R₇ is H; R₈ is selected from the group consisting of (CH₂)_(s)OH wherein s is 2 and R₁₀COOH, wherein R₁₀ is CH═CH; n is 2; q is 1; and r is
 2. 6. The compound according to claim 1, wherein R₁ is C₃ alkyl; R₃ is: —(CH₂)_(q)C₆H₄—R₇ R₅ is CH₃ (CH₂)_(p) wherein p is 1; R₆ is H; R₇ is selected from the group consisting of (CH₂)_(t)OH wherein t is 2 and R₉COOH, wherein R₉ is CH═CH; R₈ is NH₂ ; f is 0; n is 2; q is 1; and r is
 2. 7. An assay-type probe of the compound defined in claim 1, wherein said assay-type probe is labeled with non-radioactive material.
 8. The assay-type probe: according to claim 7, wherein said non-radioactive material is a fluorescent dye.
 9. The assay-type probe according to claim 7, wherein said non-radioactive material is biotin.
 10. The assay-type probe according to claim 7, wherein R₁ is C₃ alkyl; R₃ is: —(CH₂)_(q)C₆H₄—R₇ R₅ is CH₃ (CH₂)_(p) wherein p is 1; R₇ is H; R₈is NH₂; f is 0; n is 2; q is 1; r is 2; and R₆ is (CH₂)_(m)OH wherein m is 2; wherein said non-radioactive material is biotin bonded to the hydroxyl group present on R₆.
 11. The assay-type probe according to claim 7, wherein R₁ is C₃ alkyl; R₃ is: —(CH₂)_(q)C₆H₄—R₇ R₅ is CH₃ (CH₂)_(p) wherein p is 1; R₆ is H; R₇ is NH₂; n is 2; q is 1; r is 2; and R₈ is NH₂; f is 0; wherein said non-radioactive material is biotin bonded to the amino group present on R₈.
 12. The assay-type probe according to claim 7, wherein R₁ is C₃ alkyl; R₃ is: —(CH₂)_(q)C₆H₄—R₇ R₅ is CH₃ (CH₂)_(p) wherein p is 1; R₆ is H; R₇is H; n is 2; q is 1; r is 2; and R₈ is R₁₀COOH, wherein R₁₀ is an alkylene or alkenylene group having 1 to 8 carbon atoms; wherein said non-radioactive,e material is biotin bonded to the carboxyl group present on R₈.
 13. An assay-type probe of the compound defined in claim 1, wherein said assay-type probe is labeled with radioactive material.
 14. The assay-type probe according to claim 13, wherein said radioactive material is a radioactive isotope selected from the group consisting of ¹⁸F, ¹⁹F, tritium, and ¹²⁵I.
 15. A pharmaceutically,y acceptable salt of compound defined by the formula:

wherein R₁ is selected from the group consisting of C₁-C₈ alkyl; R₂ is of the formula:

wherein n is an integer ranging from 1 to 8; R₅ is H or CH₃ (CH₂)_(p), wherein p is an integer ranging from 1 to 7; and R₆ is H; (CH₂)_(m)H; or (CH₂)_(m)OH, wherein m is an integer ranging from 1 to 8; R₃ is selected from the group consisting of: —(CH₂)_(q)C⁶H₄—R₇

wherein q is an integer ranging from 1 to 8; D is selected from the group consisting of NH, S, and O; wherein R₇ is selected from the group consisting of H, OH, NH₂, R₉COOH, wherein R₉ is an alkylene or alkenylene group having 1 to 8 carbon atoms, and (CH₂)_(t)OH, wherein t is an integer ranging from 1 to 8; R₁₁ is selected from the group consisting of —CH₂COOH and —CH₂—CONH(CH₂)_(w)MHZ, wherein w is an integer ranging from 1 to 2 and Z is selected from the group consisting of hydrogen and acetate; and R₄ is of the formula:

wherein R₈ is selected from the group consisting of H; OH; (CH₂)_(f)NH₂ wherein f is selected from the group consisting of 0 and an integer ranging from 1 to 8; (CH₂)_(s)OH, wherein s is an integer ranging from 1 to 8; and R₁₀COOH, wherein R₁₀ is an alkylene or alkenylene group having 1 to 8 carbon atoms; and r is an integer ranging from 1 to
 8. 16. The compound according to claim 15, wherein R₁ is C₃ alkyl; R₃ is: —(CH₂)_(q)C₆H₄—R₇ R₅ is CH₃ (CH₂)_(p) wherein p is 1; R₆ is (CH₂)_(m)OH wherein m is 2; R₇ is H; R₈ is NH₂; f is 0; n is 2; m is 2; q is 1; and r is
 2. 17. The compound according to claim 15, wherein R₁ is C₃ alkyl; R₃ is: —(CH₂)_(q) C₆H₄—R₇ R₅ is CH₃ (CH₂)_(p) wherein p is 1; R₆ is H; R₇ is NH₂; R₈ is NH₂; f is 0; n is 2; q is 1; and r is
 2. 18. The compound according to claim 15, wherein R₁ is C₃ alkyl; R₃ is: —(CH₂)_(q)C₆H₄—R₇ R₅ is CH₃ (CH₂)_(p) wherein p is 1; R₆ is H; R₇ is H; R₈ is NH₂; f is 0; n is 2; q is 1; and r is
 2. 19. The compound according to claim 15, wherein R₁ is C₃ alkyl; R₃ is: —(CH₂)_(q)C₆H₄—R₇ R₅ is CH₃ (CH₂)_(p) wherein p is 1; R₆ is H; R₇ is H; R₈ is selected from the group consisting of (CH₂)_(s)OH wherein s is 2 and R₁₀COOH, wherein R₁₀ is CH═CH; n is 2; q is 1; and r is
 2. 20. The compound according to claim 15, wherein R₁ is C₃ alkyl; R₃ is: —(CH₂)_(q)C₆H₄—R₇ R₅ is CH₃ (CH₂)_(p) wherein p is 1; R₆ is H; R₇ is selected from the group consisting of (CH₂)_(t)OH wherein t is 2 and R₉COOH, wherein R₉ is CH═CH; R₈ is NH₂; f is 0; n is 2; q is 1; and r is
 2. 21. A pharmaceutical composition which comprises a compound of claim 1 and a pharmaceutically acceptable carrier. 